This invention relates to a vehicle dynamic characteristic control apparatus for use with a motor vehicle having controllable dynamic characteristics including yaw velocity, side velocity, roll velocity, etc. Although the invention will be described in connection with a rear wheel steering control system, it should be understood that the invention is equally applicable to other vehicle dynamic characteristic control systems including active suspension control systems and the like utilizing information on steering wheel position to control the dynamic characteristics of motor vehicles.
For example, U.S. patent application Ser. No. 07/284,414, filed Dec. 14, 1988, now abandoned, discloses a rear wheel steering control system for controlling the angle of steering of the rear wheels based on a rear wheel steering angle calculated as a function of vehicle speed and front wheel steering angle. For the purpose of calculating the front wheel steering angle, which corresponds to a deviation of the existing steering wheel position from a steering wheel neutral position indicating a driver's demand for straight ahead driving, the rear wheel steering control system includes a steering wheel position sensor for sensing the existing steering wheel position and a steering wheel neutral position sensor for producing a steering wheel neutral position signal having a first level when the steering wheel position is within a predetermined range and a second level when the steering wheel position is out of the predetermined range. The rear wheel steering control system estimates the steering wheel neutral position by calculating an average value of the steering wheel position sensed when the steering wheel neutral position signal changes from the second level to the first level and the steering wheel position sensed when the steering wheel neutral position signal changes from the first level to the second level. This estimation is made immediately when the rear wheel steering control system is powered on.
With such a rear wheel steering control system, however, the estimated steering wheel neutral position will be deviated to a great extent from a correct neutral position, causing the rear wheels to be turned in response to a driver's demand for straight ahead driving when a failure occurs in the steering wheel position sensor or the circuit associated therewith to maintain the steering wheel neutral position signal at one of the first and second levels.